Use of highly efficient working media for heat engines

ABSTRACT

The invention relates to a heat engine for performing an organic Rankine cycle (ORC) which comprises an evaporator, an engine, a condenser and a circuit comprising a fluid working medium, wherein the working medium has a critical pressure (p c ) between 4000 kPa and 6500 kPa, preferably between 4200 kPa and 6300 kPa, the working medium has a critical temperature (T c ) between 450 K and 650 K, preferably between 460 K and 600 K, the working medium has a molar mass between 50 g/mol and 80 g/mol, preferably between 60 g/mol and 75 g/mol, and the gaseous working medium partially condenses out during adiabatic expansion. The invention further relates to the use of a working medium having a critical pressure (p c ) between 4000 kPa and 6500 kPa, preferably between 4200 kPa and 6300 kPa, having a critical temperature (T c ) between 450 K and 650 K, preferably between 460 K and 600 K, and having a molar mass between 50 g/mol and 80 g/mol, preferably between 60 g/mol and 75 g/mol, in a heat engine, wherein the gaseous working medium partially condenses out during an adiabatic expansion in an organic Rankine cycle (ORC).

The invention relates to a heat engine for performing an organic Rankinecycle (ORC) which comprises an evaporator, an engine, a condenser and acircuit comprising a fluid working medium and to the use of a workingmedium for a heat engine.

There is a great demand in the chemical industry for using low energywaste heat streams generated at a temperature range from 80° C. to 250°C.

To optimize existing site integration systems, and with a view toimproving energy efficiency and reducing CO₂ emissions, one promisingoption is the conversion of these not as yet utilized waste heat streamsinto electricity through the use of combined heat and power (CHP). Thisemploys heat engines such as are disclosed in DE 10 2009 024 436 A1, DE10 2011 076 157 A1 and EP 1 016 775 A2 for example. The latter two heatengines employ water/steam as the working medium. The disadvantage ofthese is that they operate at relatively high temperatures.

The problem of high operating temperatures of steam processes has beenovercome by the use of ORC technology since this technology employsorganic fluids rather than steam as the working medium.

ORC stands for organic Rankine cycle “organischer Rankine-Kreisprozess”bedeutet. An ORC process is a thermodynamic cycle for converting heatinto mechanical work using an organic working medium.

An ORC process is a simple thermodynamic cycle in which the workingmedium is evaporated and optionally superheated by supplying heat at ahigh pressure level. The superheated vapour undergoes expansion coolingto a lower pressure in an expander (in particular an engine such as apiston engine or a turbine) thus performing work. The work may bedirectly mechanically utilized or is converted into electrical currentusing a generator. The vapour exiting the expander may still be in thesuperheated state or may already be decompressed to such an extent thatit occupies the wet vapour region so that some of it is already in theliquid state. Complete liquefaction takes place in the condenser. Here,the electricity-generating cycle is operated not with water but with anorganic working fluid which can utilize the heat generated at a lowtemperature level with greater thermodynamic efficiency.

The working medium employed thus has a key role since the optimalinteraction between the working medium and the process configuration hasa determining influence on the efficacy and thus on the efficiency ofthe entire process. For example, the working medium influences the plantconfiguration. Optimal selection of a working medium can enhance theutilization of the heat source and the efficiency of the plant.

Suitable working media for ORC processes include especially(hydro)chlorofluorocarbons and hydrocarbons and also mixtures of fluids(hydrocarbons and water, (hydro)fluorocarbon mixtures) and organicsilicon components. The existing industrially realized prior art employsnot only hydrocarbons such as pentane, but also siloxanes such asoctamethyltrisiloxane or chlorinated hydrocarbons such as R134a orR245fa (Quoilin, S., Lemort, V., Technological and Economical Survey ofOrganic Rankine Cycle, 5th European Conference Economics and Managementof Energy in

Industry, Vilamoura, Portugal, 14.04.-17.04.2009). A heat engineutilizing such ORC technology is disclosed, for example, in EP 1 174 590A2 where pentane is used as the organic working fluid, i.e. as theworking medium.

The disadvantages of the prior art working fluids include possiblehazards to the environment (CFCs: harmfulness to the ozone layer andglobal warming) and to workplace safety (hydrocarbons: flammability,explosion prevention) and also thermodynamic limitations due toinsufficient optimization of plant design and fluid properties.

For certain vapour-expansion engines (piston engines) there are nooptimized working fluids yet in existence that may be employed in thetemperature range from 80° C. to 250° C.

The fluorinated hydrocarbons are some of the most extensively describedworking media. A substantial advantage of these substances lies in theirphysical properties. For instance these substances are generally notflammable and nontoxic. The disadvantage of such substances is that theboiling point of fluorinated hydrocarbons is generally very low sincesaid substances were usually developed as coolants and are thus of onlylimited suitability for use in an ORC system at relatively high usetemperatures.

A further large group of ORC working media are hydrocarbons, for exampletoluene, pentane and isobutane. Hydrocarbons are very well known assuitable ORC working media and are employed in ORC engines. However,when utilizing these media their properties must be taken into account.The main disadvantage of these substances is that they are usuallyflammable and hazardous to the environment. Said substances generallyalso have a highly deleterious effect on climate.

As an example of a prior art ORC application, ethanol is currently usedin an ORC vapour engine from DeVeTec GmbH as the most efficient workingmedium in a temperature range starting at about 250° C.

However, since industrial waste heat streams are often at a temperaturelevel between 80° C. and 250° C. an ethanol-based ORC process cannot beoperated economically here.

In the light of this prior art the problem addressed by the invention isthat of providing a working fluid for an organic Rankine cycle (ORC)comprising a vapour-expansion engine using waste heat streams fromDeVeTec GmbH in extended temperature ranges between 80° C. to 250° C.,in particular from 80° C. to 200° C., particularly preferably from 80°C. to 150° C. This broad temperature range is a result of the differenttemperature levels of the waste heat streams. While offgases frombiogas, biomass or mine gas combustion are present at temperatures inthe region of 450° C., the industrial sphere is host to many lowertemperature streams in the range from 100° C. to 200° C. which can nolonger be utilized in many chemical sites but whose potential can beenhanced via an ORC cycle. Different working fluids are thus utilizeddepending on the application.

In addition to suitable thermodynamic properties (inter glia thermalstability, enthalpy of vapourization, vapour pressure and heat capacity)the working medium must meet further requirements such as low toxicityand low environmental impacts (for example with regard to innocuousnesstowards the ozone layer and climate) and must not be flammable norcorrosive towards components of the heat engine.

A further problem addressed by the invention is that of providing aworking medium employable with heat engines at low temperatures with ahigh degree of efficiency. The working medium shall simultaneouslyexhibit good environmental compatibility, in particular in terms ofharmfulness towards the ozone layer and climate. The working mediumshould further effect as little attack and corrosion as possible on thecomponents of such a heat engine. The working medium shall moreover beas nonhazardous as possible in its application, i.e. should exhibit thelowest possible flammability and present no risk of explosion.

Further problems addressed by the present invention and not mentionedexplicitly will become apparent from the overall context of thefollowing description, examples and claims.

These and other problems not explicitly mentioned but readily derivableor discernible from the above context discussed in the introductionhereof are solved by a heat engine having all the features of claim 1and by a method having all the features of claim 9. Protection foradvantageous developments of the inventive method according to claim 1is sought in subclaims 2 to 8. Protection for an advantageousdevelopment of the inventive heat engine according to claim 9 is soughtin subclaims 10 to 15.

The problems addressed by the present invention are solved by a heatengine for performing an organic Rankine cycle (ORC) which comprises anevapourator, an engine, a condenser and a circuit comprising a fluidworking medium, wherein the working medium has a critical pressure (pc)between 4000 kPa and 6500 kPa, preferably between 4200 kPa and 6300 kPa,the working medium has a critical temperature (Tc) between 450 K and 650K, preferably between 460 K and 600 K, the working medium has a molarmass between 50 g/mol and 80 g/mol, preferably between 60 g/mol and 75g/mol, and the gaseous working medium partially condenses out duringadiabatic expansion.

It may be provided that upon adiabatic expansion during a work cycle ofthe ORC process 1% to 30% of the mass of the working medium condensesout, preferably 10% to 20% of the mass of the working medium condensesout.

These property ranges of the working medium ensure good functioning ofthe ORC process and the heat engine with a high degree of efficiency.

It may further be provided with particular preference according to theinvention that the working medium is cyclopentene or at least one alkylformate or a mixture thereof, preferably methyl formate and/or ethylformate.

These substances are particularly suitable as working media for theintended use as is shown in detail hereinbelow.

A development of the invention proposes that the heat engine is anexpansion machine which preferably comprises a vapour expansion enginecomprising pistons as the engine or which comprises at least one turbineas the engine.

In the context of the present invention the engine may thus be realizedeither as a piston engine or as a turbine. Other types of heat enginesmay also be employed as the engine provided they are capable ofconverting the expansion work of the working medium into mechanical workutilizable outside the process. It is thus also possible to employ arotary engine.

A vapour expansion engine having reciprocating pistons is particularlypreferred in accordance with the invention since the wet behaviour ofthe working medium makes it possible to eschew a recuperator and theconversion of the ORC process may thus be carried out in particularlycost-effective fashion.

The mechanical work delivered by the engine may be directly mechanicallyutilized or converted into electrical current using a generator.

It may also be provided that a pump is disposed between the condenserand the evapourator in the circuit of the heat engine, said pumpallowing the fluid working medium to be conveyed from the condenser tothe evapourator.

This ensures that the ORC process may be readily started up.

A particularly preferred embodiment of the invention may provide thatthe circuit of the heat engine does not comprise a recuperator.

The eschewal of a recuperator (heat exchanger) is made possible by theworking media according to the invention. This makes the heat enginesimpler and more cost-effective to set up.

It may also be provided with preference that the erosion rate of theworking medium towards unalloyed steel is less than 0.05 mm/a at 150° C.and/or that the erosion rate of the working medium towards alloyed steel(1.4571) is less than 0.005 mm/a at 150° C.

This ensures that long-term operation of the heat engine with theworking medium is possible.

It may further be provided that the working medium exhibits noendothermic or exothermic reactions or first or second order phasetransitions in the temperature range between 70° C. and 200° C. whensubjected to temperature changes over time, preferably not even whensubjected to tenfold repetition of a temperature/time profile between70° C. and 200° C.

Such phase transitions might disrupt the ORC process.

The problems addressed by the invention are also solved by the use of aworking medium having a critical pressure (pc) between 4000 kPa and 6500kPa, preferably between 4200 kPa and 6300 kPa, having a criticaltemperature (Tc) between 450 K and 650 K, preferably between 460 K and600 K, and having a molar mass between 50 g/mol and 80 g/mol, preferablybetween 60 g/mol and 75 g/mol, in a heat engine, wherein the gaseousworking medium partially condenses out during adiabatic expansion withina cycle of the ORC process.

The problems addressed by the invention are preferably solved by the useof alkyl formates or cyclopentene or mixtures thereof as the workingmedium in a heat engine.

It may be provided that methyl formate and/or ethyl formate are employedas the alkyl formate, preference being given to employing methyl formateor ethyl formate as the working medium in the heat engine.

The process according to the invention is easy to implement and thuscost effective in its realization.

As a further criterion the use of mixtures may be highly advantageousfor reducing the energy losses during heat transfer since theevaporation thereof does not occur at constant temperature.

Uses according to the invention may preferably provide that the heatengine is operated with an ORC process. The substances and substanceclasses at issue are particularly suitable for ORC processes.

It may also be provided that the heat engine employed is an expansionmachine, preferably a vapour expansion engine comprising pistons or atleast one turbine as the engine.

It may finally also be provided that the heat engine is operated with aheat source in a low-temperature range between 80° C. and 200° C.,preferably between 80° C. and 150° C.

The working media intended for use are particularly suitable for the lowtemperature range.

One fundamental finding of the is that working media having suitablephysical properties in terms of critical pressure, suitable boilingpoint and suitable behaviour during adiabatic expansion, namely partialcondensation, may be used to carry out an ORC process in a heat enginewith which low-temperature offgas streams too may be utilized forconversion into electricity without the occurrence of other deleteriouseffects.

Accordingly, endeavours in the context of the present invention led tothe development of novel, efficient working fluids/working media for aheat engine.

In order to achieve the objective of efficient utilization of wasteheat, endeavours in the context of the present invention led to theidentification and development of working media (i.e. working fluids)for low temperature applications which not only achieve maximumthermodynamic efficiency but are also optimal from safety andenvironmental aspects.

Of central importance for the suitability of a chemical substance as aworking medium are in particular the following material data/measuredparameters which are characterizable by the derivable parameters andrelationships that they intimate.

1. Vapour pressure:

-   -   characterizable by the temperature and pressure range of the        process (low- or high-temperature)    -   derivation of the gradient of the saturated vapour line in the        T-S diagram from Δh_(Lv), C_(p), (2 methods) (wet or dry fluid,        condensation during adiabatic expansion)    -   large enthalpy of vaporization (large pressure ratio of upper to        lower process pressure)    -   derivation of optimal process conditions

2. Heat capacity:

-   -   derivation of the gradient of the saturated vapour line in the        T-S diagram from Δh_(Lv), C_(p), (heat transfer area capital        expenditure costs)

3. Thermal and chemical stability:

-   -   high thermal and chemical stability (in contact with steel,        lubricants, seals, air, water)

4. Viscosity:

-   -   general applicability, pump work, heat transfer (heat exchanger        capital expenditure costs)

5. Corrosivity:

-   -   low propensity for corrosion

6. Criticality data:

-   -   critical temperature, critical pressure and critical volume

7. Thermal conductivity:

-   -   heat transfer

8. Density:

-   -   heat transfer    -   apparatus dimensioning (high vapour density→low specific        volume→small streams)

9. Molar mass:

-   -   It is a tendency that: the greater the molecules the higher the        critical volume of the critical temperature and the poorer the        high-temperature resistance

Δ_(Lv) is the enthalpy of vaporization at constant volume, c_(p) is theheat capacity at constant pressure, T_(c,Fluid) is the criticaltemperature of the working medium, T_(process) is the processtemperature, T is the temperature and S is the entropy.

One particular advantage of a heat engine filled with a working mediumaccording to the invention (for example the piston expansion engine fromDeVeTec GmbH) is that so-called “wet” working fluids, which may bedecompressed into the wet vapour region, may be employed. Recuperationis not necessary for such a fluid and the engine for performing theprocess may therefore be markedly simplified.

Hereinbelow, exemplary embodiments of the invention and diagramsrelating to the invention are elucidated by reference to eightschematically represented figures and diagrams without any intention torestrict the invention. Dabei zeigt:

FIG. 1 shows a simplified schematic representation of an ORC process/aheat engine for implementing a process according to the invention;

FIG. 2 shows an ideal-type representation of the changes of state forwet, dry and isentropic fluids in the ORC process in atemperature-entropy diagram;

FIG. 3 shows a schematic representation of a setup for determining thevapour pressure of suitable working media;

FIG. 4 shows the temperature/time profile for a calorimetric measurement(DSC) for analyzing suitable working media;

FIG. 5 shows a vapour pressure/time diagram for determining the thermalstability of 1-propanol at 195° C. to 180° C.;

FIG. 6 shows a vapour pressure/time diagram for methyl formate at 150°C.;

FIG. 7 shows a vapour pressure/time diagram for ethyl formate at 150°C.;

FIG. 8 shows cyclic differential thermal analysis diagrams (DSC curves)for ethyl formate.

FIG. 1 shows a simplified schematic representation of an ORC process forimplementing a process according to the invention, i.e. an ORC process,such as is carried out in a heat engine according to the invention.

The ORC process depicted is a simple thermodynamic cycle in which aworking medium is evaporated and optionally superheated at a highpressure level by supplying heat. The superheated vapour undergoesexpansion cooling to a lower pressure in an engine (for example aturbine or piston engine) thus performing work. The vapour exiting theexpander may still be in the superheated state or may already bedecompressed to such an extent that it occupies the wet vapour region sothat some of the working medium is already in the liquid state. Completeliquefaction takes place in the condenser. Here, theelectricity-generating cycle is operated not with water but with anorganic working fluid which can utilize the heat generated at a lowtemperature level with greater thermodynamic efficiency.

A parameter of central importance is the vapour pressure of thecomponents which firstly permits general classification for the low- orhigh-temperature range. Efficient working fluids make it possible torealize, for a given temperature of the heat source and the heat sink,the greatest possible pressure ratio between the upper and lower processpressure. This requirement may readily be shown in a logarithmicrepresentation of the vapour pressure via the negative reciprocalabsolute temperature as is shown in FIG. 2. Since the gradient of thevapour pressure curve in the Raoult diagram is proportional to theenthalpy of vaporization in accordance with the Clausius-Clapeyronequation, working media having large enthalpies of vaporization promiseadvantages on account of the greater expected pressure ratio in theexpander. Together with the heat capacity there are also methods ofestimation that allow predictions to be made regarding the fluid type(wet, dry or isentropic).

The changes of state of the working fluid in the cycle may be depictedin the temperature (T) entropy (S) diagram. FIG. 2 shows the advancementof the process for different fluid types in the T-S diagram with thesimplification that the fluids are decompressed in isentropic fashion.The working fluids may be categorized according to the path of thesaturation line and the dew line into wet (negative gradient dew line),dry (positive gradient dew line) and isentropic (vertical dew line)working fluids. The substantial difference when using these differentfluid types in the ORC process lies in the state of the vapour after thedecompression. For wet and isentropic fluids the vapour is in thesuperheated state only to a very limited extent, if at all, after thedecompression, i.e. the fluid is decompressed into the wet vapour regionso that liquid droplets are already present. In the case of the dryfluids a superheated vapour is present which is at a temperature higherthan the condensation temperature. Depending on the proportion of heatin the superheated steam it may be necessary in the case of turbineutilizations to use this unutilized heat for warming the cold fluidafter the pressure increase in order to achieve improved efficienciesfor the process. Process costs may simultaneously be increased by about30% due to the use of the additional heat exchanger.

In certain cases using wet fluids as ORC media is advantageous and thuspreferable since said fluids make it possible to eschew a recuperator(heat exchanger). The further required properties (see above) only comeinto play after this fundamental requirement has been met but are thenno less important. The most important requirements include thermal andchemical stability, low viscosity, no corrosivity, no toxicity, easyhandleability (explosion limits outside operating conditions, noflammability).

In order to operate the ORC process in economic fashion, preferenceamong the potential working media is given to wet/isentropic behaviourin order that a recuperator may be eschewed. A medium is referred to asa wet fluid when the gradient of the dew line in the T-S diagram isnegative (FIG. 2). This results in the formation of wet vapour uponisentropic decompression starting from the dew line. When the dew lineis vertical the medium is referred to as isentropic and when thegradient is positive the medium is referred to as dry.

In order to evaluate the thermodynamic suitability of new working mediain the ORC process a model of the cycle was constructed in the “AspenPlus” computer simulation program which allows the thermal efficiency tobe calculated as a function of the medium employed and the temperatureof the available heat source.

The following boundary conditions derived from the apparatuses employedby DeVeTec apply to the simulation:

-   -   efficiency of the pump: 65%    -   maximum pressure: 35 bar    -   efficiency of the expansion machine: 88%    -   final conditions of the expansion: either 1.1 bar or 35° C.    -   total condensation without supercooling

The maximum temperature in the evaporator is accordingly a degree offreedom. The simulations were performed for various temperatures: 100°C., 150° C., 200° C. and 250° C. The thermal efficiency of the processwas evaluated for the various conditions.

The efficiency is generally defined as:

$\eta = \begin{matrix}Q_{useful} \\Q_{supplied}\end{matrix}$

-   η—efficiency-   Q_(useful) —useful energy-   Q_(supplied)—supplied energy

In the case of the organic Rankine cycle process (ORC process) theutility is the output of the expansion machine. The input is composed ofthe power of the pump and the supplied heat.

Evaluation of the simulations makes it possible to compile a list of thetheoretically achievable efficiencies for the various operatingconditions. Ethanol was defined as the reference medium. Theparticularly suitable working media found in the context of the presentinvention were compared with the working medium ethanol for varioustemperatures. In general terms it should be noted that the choice ofworking medium is dependent on the heat source available. Depending onthe evaporator temperature certain working media are more or lesssuitable for use as the working medium in a heat engine.

TABLE 1 Efficiency at the following maximum temperatures 200° C. 150° C.100° C. methyl formate 22.65 19.82 13.72 2,3-dihydrofuran 20.98 16.509.46 tetrahydrofuran 19.42 14.60 7.03 cyclopentene 20.76 16.78 10.30ethyl formate 20.46 16.19 9.34 ethanol 18.20 13.02 4.75

Compared to ethanol there is a marked improvement in efficiency at loweruse temperatures. Further investigations were carried out for the use ofthe selected particularly preferable substances. In particular, thestability of the substances at the use temperature was analyzed.

The vapour pressure is the pressure established when a vapour is inthermodynamic equilibrium with the associated liquid phase in a sealedsystem. The vapour pressure increases with increasing temperature and isa function of the substance/mixture present. When the vapour pressure ofa liquid is equal to the ambient pressure in an open system the liquidbegins to boil.

The vapour pressure is one of the crucial substance properties for thedesign and operation of an ORC plant. Due to the operating conditionsdefined for the vapour engine the vapour pressure of a suitable liquidshould be below 35 bar.

The vapour pressures of the working media are determined in a sealed andtemperature-controlled high-pressure autoclave. This comprises heatingthe liquid and measuring the pressure at the particular temperaturesetting. The more accurate the measurement of these two values thebetter the determined vapour pressure data. Calculations may beperformed with “Aspen Plus” for comparison with the literature values.In the case of deviations in the data, in-house measurements of thevapour pressure may then be performed.

Specific heat capacity indicates the amount of heat that needs to besupplied to a kilogram or a mole of a particular substance to raise itstemperature by 1 Kelvin.

These substance-specific data are necessary in particular for the designof the heat engineering components of an ORC system. Experimentaldetermination of the data is performed in a calorimeter. Heat capacityis generally measured using DSC (differential scanning calorimetry).

Viscosity is a measure of the resistance of a fluid to deformation andinfluences heat transfer and pump performance in an ORC system. Forcomparison at 20° C. water has a viscosity of about one mPas, edibleoils have a viscosity of about 100 mPas and honey has a viscosity ofabout 1000 mPas. The lower the viscosity the more mobile a liquid andthe quicker said liquid can flow under constant conditions. Suitable ORCworking media should therefore have a low viscosity of less than 10 mPasat 20° C.

The chosen working media all have a rather low viscosity which iscomparable to the viscosity of water (about 1 mPas at 20° C.). In theregion above about 100° C. which is of interest for an ORC system theviscosities of the preselected working media hardly differ from oneanother anymore.

One of the further important substance properties for the design of athermodynamic cycle is the density of the liquid and gaseous phase ofthe working medium.

The density of the working media is essential to the design of thecirculation pumps. Volume flow is converted into mass flow using thedensity of the substances.

The data for the cited physical parameters of the various substances areobtainable from the literature and/or from databases concerning theworking media analyzed.

The enthalpy of vaporization is the amount of heat required to effectthe transition of a liquid from the liquid into the gaseous state. Theconverse process in which the gaseous medium is reliquefied gives offthe heat of condensation. Both parameters are of great importance for athermodynamic cycle in which a liquid is continually evaporated andrecondensed.

The enthalpy of vaporization may be obtained from the literature or,similarly to the heat capacity, measured by calorimetric methods (forexample by DSC).

The vapour pressure is one of the most important physical substanceproperties of a working medium. Designing an ORC system and validatingthe simulation data require accurate knowledge of the vapour pressurecurve. Equipment allowing accurate measurement in an absolute pressurerange from 0 bar to 100 bar and at temperatures from 20° C. to 400° C.was constructed for the experimental determination of said curve. Sinceaccurate measuring means for such a large measurement range are notavailable the equipment was divided into three measurement regions.Table 2 which follows summarizes the permissible operation data for theindividual autoclaves.

TABLE 2 Design parameters for the vapour pressure measuring apparatusautoclave 1 autoclave 2 autoclave 3 temperature range 20-150° C. 20-250°C. 20-350° C. pressure range 0-2 bar 2-50 bar 50-100 bar volume 100 ml100 ml 100 ml

Measurement accuracy was enhanced by using pressure sensors (fromEndress & Hauser) calibrated for the relevant pressure and temperaturerange. The autoclaves were heated using an electric heating collar.Temperature control was effected by measuring the temperature in theindividual autoclave and in the heating collar using precise Ni-Crtemperature sensors and comparing these temperatures with one another.The autoclaves were sealed using special copper washers and copperpaste. The apparatus and the conduits were fully insulated to reduceheat losses and achieve improved controllability. The integrated vacuumpump makes it possible to obtain measurements under high vacuum. Thevacuum is also required in particular when changing the fluids forcleaning purposes and for purging the measuring means with nitrogen foravoiding explosive atmospheres. Readings were acquired using anautomatic data acquisition means with a sampling rate of one second forthe entire duration of the test. A basic schematic construction of themeasuring means is shown in FIG. 3.

Startup and calibration of the measuring means was carried out withethanol and water, ethanol being suitable for the pressure range up to60 bar. The vapour pressure of water was measured at pressures up to 100bar. The two substances were also chosen because the data for thesubstances are well known and may be consulted for validation of theapparatus. It was found that the deviation is below 1% of the absolutevalue and the method of measurement is therefore suitable for thefurther investigations. For the high-pressure range too the measuringmeans was sufficiently validated with the data for water.

The actual suitability as a working medium depends not only on themaximum obtainable efficiency but also to a substantial extent on thelong-term stability of the substances when in use. Thermal decompositionof the substances can result in undesired byproducts which can lead, forexample, to a reduction in the vapour pressure or to corrosion of thematerials employed in the heat engine. In the first screening operationthe working media were subjected to short-term stress and analyzed interms of a plurality of criteria. Four substances were selectedtherefrom for further tests. The second test phase comprised carryingout extensive corrosion and material compatibility tests. The third testphase comprised carrying out long-term tests.

The working media were subsequently tested in a heat engine underrealistic conditions.

Knowledge of the thermal stability of a substance is generallyindispensable. An untested substance may suffer a loss of quality andgive rise to unforeseeable hazards due to excessive temperatures duringproduction, storage and transport. It is an important feature of theworking media sought that no undesired decomposition products aregenerated during use which could endanger the operation of the plant.

Thermal stability was determined using the following principle ofmeasurement:

The working media were charged into an autoclave at room temperature andinertized with nitrogen. The temperature of the medium was subsequentlyincreased up to a maximum use temperature and sustained for a prolongedperiod. The vapour pressure of the substance was initially determined atroom temperature and compared with literature values. This was followedby continuous determination of the vapour pressure as a function oftemperature and long-term measurement at the maximum temperature. Oncompletion of the test the working medium was cooled and analyzed by gaschromatography.

The gas chromatograph (GC) allows the composition of substance mixturesto be determined. This results in a chromatogram in which all substancesare unambiguously assigned. The measurement is performed for anuntreated laboratory-tested substance. This makes any decompositionproducts formed unambiguously determinable. The measurement makes itpossible to determine not only the type of byproducts but also thepercentage fraction thereof.

A further method for determining thermal stability is differentialscanning calorimetry (DSC). This method was used to determine stabilityover a plurality of cycles.

DSC comprises heating two sealed crucibles (first crucible containingabout 10 mg of sample and second empty crucible as reference) at apredetermined heating rate (10 Kelvin/minute in this case) up to atarget temperature (up to 200° C. in this case). Both crucibles aresubjected to the same temperature program. The energy absorption ordecrease is analyzed during heating. The energy balance changes incomparison to the empty sample depending on the the heat capacity of thesample or exothermic and endothermic processes in the sample such asmelt or vaporization. Once heating is complete the sample is held at aconstant maximum temperature. For a thermally stable substance no energychanges occur during this time. Decomposition of the substance isobserved via a change in the energy absorption or energy decrease.

FIG. 4 shows the employed temperature/time profile for the DSC. Over thetime period from 0-20 minutes the temperature is increased as a constantheating rate and energy is correspondingly absorbed. In the rangebetween 20-50 minutes the temperature is kept constant. For a stablemedium no absorption or emission of energy occurs. Between 50-70 minutesthe sample is cooled down again and the temperature is reduced with acorresponding energy decrease.

The reproducibility of the measurement was confirmed by carrying out aplurality of cycles per medium. This is because the decompositionproducts may also arise only after a prolonged operating time and aplurality of cycles.

Since the selected working media could exhibit corrosive behaviourtowards the employed materials of the ORC engine, extensiveinvestigations into corrosion behaviour were carried out. To this end,both metallic and nonmetallic (largely elastomeric materials of theseals) materials were defined and investigated in conjunction with theindividual media.

For metallic materials samples having defined dimensions were prepared.To determine the erosion rates the metal strips were weighed andcompletely submerged in the respective fluid in an autoclave. Theautoclaves were sealed, inertized and brought to a defined temperatureand held at this temperature over a prolonged period. The metal stripswere subsequently removed again, cleaned and weighed to determine theerosion rate. To determine any local corrosion the individual sampleswere examined by microscopy.

To investigate the corrosion behaviour of the working media towardsnonmetallic materials the following tests were conducted:

Long-term thermal stability is crucial for trouble-free operation of anORC system. However, working media may be decomposed by use at hightemperatures. The stability of a novel working medium must therefore beestablished prior to its use. The relevant tests were carried out in ahigh-pressure autoclave with the objective of determining the maximumuse temperature of each medium. The test temperature and test pressurewere measured. Decomposition of the fluid also results in a change invapour pressure. This change may in turn be observed by reference to themeasured values. The decomposition products were analyzed in a gaschromatograph and compared with the starting product.

FIG. 5 depicts the investigation of the thermal stability of 1-propanolat 195° C. and 180° C. The measured vapour pressure (upper curve)increases with time at constant temperature (lower curve) at varioustemperatures between 195° C. and 180° C. This shows that 1-propanol isnot stable at these use temperatures. Below 180° C. the vapour pressurebecomes too low (less than 20 bar) to be usefully employable as theworking medium in a heat engine. In the experimental setup of FIG. 6methyl formate was stored at a temperature (upper curve) of about 150°C. The vapour pressure (lower curve) remains constant and the fluid maytherefore be described as stable at this temperature.

All potential working media were investigated for use temperatures offrom 150° C. to 200° C. in this fashion. Ethyl formate also exhibitssimilar behaviour to methyl formate (FIG. 7). At a use temperature of175° C. this fluid undergoes slight decomposition over time. At a usetemperature of 150° C. (upper curve in FIG. 7) it remains stable, i.e.the vapour pressure (lower curve in FIG. 7) does not increase.

The working media methyl formate, ethyl formate and cyclopentene areparticularly advantageous on account of these investigations forexample. The extended investigations tested the thermal stability of thepreselected working media in a longer test of two months in duration.

The working media tested were stored in high-pressure autoclaves at anoperating temperature of 150° C. After the test the decomposition rateof all samples was investigated by GC analysis to determine thermalstability. The results of this analysis are summarized in table 3. Themaximum decomposition of the working media methyl formate, ethyl formateand cyclopentene was about 2% and is therefore in an industriallyacceptable range.

TABLE 3 Degree of purity and decomposition of the working media aftersubjection to thermal stress for 2 weeks. before test after test percentdecomposition methyl formate 98.62% 97.21% 1.43% ethyl formate 97.28%95.16% 2.18% cyclopentene 98.51% 98.31% 0.20%

The following investigations sought to test the extent to which theworking media employed exhibit corrosive behaviour towards the typicalmaterials employed in heat engines. The following materials were testedin the corrosion tests: unalloyed steel (P265GH) and alloyed steel(1.4571) including a weld seam. The materials were employed in the formof sheet-metal (90 mm×10 mm×6 mm). The test specimens were weighed in amaterials engineering laboratory and characterized by opticalmicroscopy. The test was then carried out in the abovedescribedapparatus for measuring vapour pressure. Once the samples were removedevaluation was once again performed in the materials engineeringlaboratory. The results of the first corrosion investigation are shownin table 4.

TABLE 4 Test results and evaluation of the corrosion tests workingerosion rate microscopy material medium [mm/a] findings unalloyed steel(P265GH) methyl formate 0.0301 no findings alloyed steel (1.4571) 0.0024no findings unalloyed steel (P265GH) ethyl formate 0.0384 no findingsalloyed steel (1.4571) 0.0034 no findings unalloyed steel (P265GH)cyclopentene 0.0317 no findings alloyed steel (1.4571) 0.0013 nofindings

Although the optical microscopy evaluation found no local corrosion andcracks a crack test was additionally performed on material 1.4571 usingthe penetration method. This also found no cracks. The technicalstability limit for metallic materials is given by an erosion rate of≦0.1 mm/annum. There must moreover not be any instances of localcorrosive attack since these preclude technical stability of thematerials. The two material classes tested must accordingly be ranked ashaving technical stability towards the preferred working media methylformate, ethyl formate and cyclopentene under the cited test conditionsat 150° C., i.e. the three working media are fundamentally suitable.

Cyclic stability tests carried out with the described DSC method showedno deterioration/change in the working media (methyl formate, ethylformate and cyclopentene). By way of example FIG. 8 shows the cyclic DSCcurves of ethyl formate, with methyl formate and cyclopentene alsoshowing similar curves. The upper curve once again shows the employedtemperature profile for the DSC measurement. The lower set of curvesrepresents the results of the DSC measurement. All three working mediatherefore show sufficient long-term storage.

Measurements of the efficiency of the cycle with the selected workingmedia (methyl formate, ethyl formate and cyclopentene) and theabovementioned tests determined that fluids are particularly suitablefor use in the heat engine when the critical pressure p_(c) is between4000 kPa and 6500 kPa, in particular between 4200 kPa and 6300 kPa,particularly preferably between 4700 kPa and 6000 kPa, the fluids have acritical temperature (T_(c)) between 450 K and 650 K, preferably between460 K and 600 K, particularly preferably between 475 K and 510 K, andthe fluids have a molar mass between 50 g/mol and 80 g/mol, preferablybetween 60 g/mol and 75 g/mol. Such fluids are also usable with a highdegree of efficiency at low temperatures of the offgas to be utilized/ata low temperature of the evaporator. It has been found that to simplifythe construction of the heat engine the use of a recuperator (heatexchanger) may be eschewed when a “wet” working medium is employed. Theworking medium is referred to as a “wet” working medium when the gaseousworking medium undergoes partial condensation upon adiabatic expansion.

These criteria are well met by the preferred working media methylformate, ethyl formate and cyclopentene. Thus, the critical temperature(T_(c)) of methyl formate is 487 K, that of ethyl formate is 508 K andthat of cyclopentene is 507 K. The critical pressure p_(c) of methylformate is 5998 kPa, that of ethyl formate is 4742 kPa and that ofcyclopentene is 4820 kPa. The molar mass of methyl formate is 60 g/mol,that of ethyl formate is 68 g/mol and that of cyclopentene is 74 g/mol.All three of these working media undergo partial condensation uponadiabatic expansion and it is therefore possible to eschew a recuperatorin the circuit of the ORC.

For the simulation conditions the efficiency of the inventive workingmedia in a heat engine at an offgas temperature (evaporator temperature)between 80° C. and 200° C. is superior to prior art working media forheat engines, for example ethanol. These results were confirmed byexperiment for methyl formate in an ORC engine (piston expansion engine)from Devetec GmbH. The working media according to the invention thusachieve an improvement in the efficiency of the heat engine attemperatures between 80° C. and 200° C., in particular between 80° C.and 150° C.

The features of the invention disclosed in the above description and inthe claims, figures and exemplary embodiments may be essential to therealization of the invention in its various embodiments eitherindividually or in any desired combination.

1. A heat engine for performing an organic Rankine cycle (ORC), the heatengine comprising an evaporator, an engine, a condenser and a circuitcomprising a fluid working medium, wherein the working medium is methylformate, and the heat engine is operated with a heat source at atemperature ranging from 80° C. to 150° C.
 2. The heat engine accordingto claim 1, wherein, during adiabatic expansion during the organicRankine cycle (ORC), 1% to 30% of the mass of the working mediumcondenses out.
 3. (canceled)
 4. The heat engine according to claim 1,wherein the heat engine is an expansion machine.
 5. The heat engineaccording to claim 1, wherein a pump is disposed between the condenserand the evaporator in the circuit of the heat engine, said pump allowingthe fluid working medium to be conveyed from the condenser to theevaporator.
 6. The heat engine according to claim 1, wherein the circuitof the heat engine does not comprise a recuperator.
 7. The heat engineaccording to claim 1, wherein an erosion rate of the working mediumunalloyed steel is less than 0.05 mm/a at 150° C. and/or an erosion rateof the working medium towards alloyed steel (1.4571) is less than 0.005mm/a at 150° C.
 8. The heat engine according to claim 1, wherein theworking medium exhibits no endothermic or exothermic reactions or firstor second order phase transitions in the temperature range between 70°C. and 200° C. when subjected to temperature changes over time.
 9. Aprocess, comprising operating a heat engine with methyl formate as aworking medium, wherein the heat engine is operated with a heat sourceat a temperature ranging from 80° C. to 150° C.
 10. The processaccording to claim 9, wherein, during adiabatic expansion during theorganic Rankine cycle (ORC), 1% to 30% of the mass of the working mediumis condensed out. 11-12. (canceled)
 13. The process according to claim9, wherein the heat engine is operated with an organic Rankine cycle(ORC).
 14. The process according to claim 9, wherein an expansionmachine is used as the heat engine.
 15. (canceled)